Thanks! That's right, it's a voltage and current regulator box. The cable for dedicated GUIDE AA charger output is shown there too. There is also a 12V auto cigar lighter dongle (female) in the kit, but I don't use it, relying on the steel wire if the need arises.

Also you might be noticing the open pocket under the battery pack. It can conveniently and securely hold a phone or the batt.pack while it's charging from the panel, let's say, strapped to your backpack on the trail.

Overall I'm very satisfied with the purchase. Good build quality, great functionality, right size and weight, fair price.